1. Field of the Invention
The present invention relates generally to a zoom lens system and, more particularly, to a zoom lens system suited as a projection lens for a liquid crystal projector.
2. Related Background Art
In recent years, with a spread of a liquid crystal projector for a domestic use, there has been rapidly increased demands for reducing a weight thereof, attaining down-sizing thereof and saving its energy.
First, a liquid crystal panel is being miniaturized for reducing both of the weight and the size of the liquid crystal projector. Recently, even a hi-vision oriented liquid crystal projector which pursues a high image quality is mounted with a liquid crystal panel as small as 2 inches. With the down-sizing of this liquid crystal panel, the down-sizing of a projection optical system and of an illumination optical system has been realized. Thus, the downsizing of the liquid crystal panel makes a great contribution to reducing the weight and the size of the liquid crystal projector apparatus.
Further, there is a necessity for illuminating the liquid crystal panel with the unidirectionally-polarized light beam in terms of a characteristic of the liquid crystal panel. For this reason, it is not a conventional practice that the light beams polarized in other directions are used as the illumination light. Recently, however, the conventionally unused light beams polarized in other directions can be utilized as the illumination light by a combiner. Hence, if an output of the light source is the same, it is possible to obtain a picture which is brighter by approximately 1.5 time than in the prior art. In other words, the light source output that is approximately 2/3 as small as the conventional one may suffice for obtaining the picture exhibiting the same brightness, and, therefore, utilizing the light beams polarized in other directions as the illumination light by use of the combiner greatly contributes to the saving of the energy of the liquid crystal projector.
There exists, however, an inconvenience inherent in the conventional liquid crystal projector, wherein the screen does not yet have a brightness enough to enjoy the picture in a room that is light to some extent.
Further, with the down-sizing of the above-mentioned liquid crystal panel, a liquid crystal panel magnifying power for obtaining the picture with the same size increases, resulting in such a inconvenience that the picture is further darkened.
Under such circumstances, there rapidly increases a demand for a bright projection lens in order to obtain the bright picture.
By the way, if the projection lens is set brighter, as a matter of course, the projection lens augments in size, and, besides, the number of constructive lenses increases. In addition, there is a necessity for making incident a greater amount of oblique light beams (incident from the oblique direction with respect to the optical axis) in order to secure a marginal light quantity. As a result, the aberration is hard to correct, and this in turn causes an occurrence of a comatic flare. Accordingly, in the projection lens for the conventional liquid crystal projector, generally, the F-number is on the order of 3.5 to 4.0, and the marginal light quantity rate (vignetting factor) is approximately 50%.